Coin telephone antifraud circuit



Feb. 18, 1969 Sheet Filed Jan. 28. 1966 FIG.

K H mm W P SM 7 m m C m do 0 M C wmw cl s M m 5 wwxm m C OM C L 3 D 0 02 R H M An M L wo mm L 0 0 L 0 R AE mmm m C (I FT R I 0 T 2 H x 2 T T mll/||l I 3 l I I III 2 0D MC 4 Q w m w U M 2 v A m F 3 ra 2 0 L WWW m lw B L T r G0 INVENTOR J. E. EDINGTON BY ATTOQAE) Feb. 18, 1969 J. E.EDINGTON COIN TELEPHONE ANTIFRAUD CIRCUIT Sheet FIG. 3

COMMON /SW{STCH 2 m w m a 5 l I Q m w M w E w B L +C O Feb. 18, 1969 J.z. EDINGTON 3,428,751

COIN TELEPHONE ANTIFRAUD CIRCUIT Filed Jan. 28. 1966 Sheet 3 of 5 FIG.5A

l T GUARD 2 GUAED 3 LAST GUARD DIG/7' TONE DIG/7' TONE DIG/T DIG/T TONEMUL TIFREQUENCV SIGNALS NORMAL TOUCH TONE DIAL ING SIGNAL OUTPUT FIG. 5B

FRAUD GUARD FRAUD GUARD TONE TONE rogv TOINE FRAUDULENT ATTEMPTED S/MULAT/ON OF I0 DEPOSIT B) TOUCH TONE SIGNALS United States Patent F3,428,751 COIN TELEPHONE ANTIFRAUD CIRCUIT John E. Edington,Indianapolis, Ind., assignor to Bell Telephone Laboratories,Incorporated, New York, N .Y.,

a corporation of New York Filed Jan. 28, 1966, Ser. No. 523,594

U.S. 'Cl. 179-6.3 Claims Int. Cl. H04m 17/02 ABSTRACT OF THE DISCLOSUREThis invention relates to coin operated telephones using voice-frequencytones for dialing and for indicating the value of coins inserted.Fraudulent or accidental misregistering of dial frequencies as depositfrequencies is prevented by following each dial frequency with ameasured burst of a specified guard frequency, thereby identifying alldial-originated frequencies.

This invention relates to coin operated telephones and more specificallyto coin telephones employing dial generated multifrequency signaling.

An essential feature of any coin operated telephone is a meansresponsive to the deposit of a coin for generating signals indicative ofthe value of the deposit. The most common coin deposit signaling meansis an arrangement 'wherein each deposited coin strikes a distinctivegong or chime. The sound generated thereby is converted to an electricalsignal by means of a transducer which signal is then applied to theline. The corresponding sounds heard by the operator are readilyinterpreted as representative of the particular coin deposited.

A more recent development in the field of coin deposit signaling employsa switching device that operates one or more times in response to thedeposit of each coin, each switch operation being indicative of anincrement of coin value, such as five cents, for example. Each operationof the switch energizes an oscillator that generates an oscillatory,electrical signal burst in the audio frequency range that is thenapplied to the line. The corresponding audio tone pulses are counted bythe operator to determine the value of the coins deposited. A system ofthis type is disclosed by E. :R. Andregg and L. A. Strommen in Patent3,146,312 issued Aug. 25, 1964.

When either of the above-described coin identification signal systems isused in a telephone employing a multifrequency signal generating dial,one form of which is termed a Touch-Tone dial, a possibility ofconfusion arises in distinguishing the dial signals from the coinsignals. In the Touch-Tone dial, for example, the simultaneous operationof certain combinations of dial pushbuttons results in the generation ofa single frequency tone which can be mistaken by an operator for thesingle frequency tone generated by a coin identification signaloscillator, despite a substantial frequency separation between the twosignals. Additionally, some operators find difficulty in distinguishingbetween the multifrequency tone that results from the operation of asingle dial-pushbutton and a single frequency coin signal. In someinstances other operators are unable to distinguish between Touch-Tonesignals, either single frequency or multifrequency, and the broadspectrum chime and gong coin deposit signals. Whether the simulation ofcoin deposit signals occurs through accidental or fraudulentmanipulation of the pushbuttons of a multifrequency type dial, the sameundesirable result occurs-providing telephone service without charge.

Accordingly, an object of the invention is to reduce the possibility ofconfusion between multifrequency dial signals and electrically generatedcoin signals in coin operated telephones.

Patented Feb. 18, 1969 Another object is to reduce the possibility ofconfusion between multifrequency dial signals and mechanically generatedcoin signals in coin operated telephones.

A further object is to reduce the incidence of fraud in the use of coinoperated telephones.

These and other objects are achieved in accordance with the principlesof the invention in one illustrative embodiment wherein a coin operatedtelephone is provided with a guard tone oscillator responsive only tothe release from an operated condition of any of the pushbuttons of anotherwise conventional Touch-Tone dial to the end that each normallygenerated multifrequency dialgenerated pulse is followed immediately bya distinctive single frequency guard tone, thereby eliminating anypossible confusion between dial signals and coin deposit signals. Inorder to avoid any possible interference with the normal operation ofthe central ofiice multifrequency receiving equipment, the inventionprovides for the automatic removal of the guard tone whenever the dialis actuated at the time that the guard tone is present.

In accordance with one aspect of the invention, means are provided forpreventing the customer from hearing the guard tone thus deterring himfrom interfering with guard tone generation and further, preventing anytendency the customer might otherwise have to reduce his normal dialingrate as the result of delaying the operation of each dial pushbuttonwhile awaiting the termination of the guard tone signal.

In order further to mark the guard tone with unmistakablecharacteristics, tone duration is employed as a distinctive qualityinasmuch as differences in frequency and amplitude are more difficultfor the average operator to detect. Specifically, guard tone duration ison the order of one full second provided that the tone is not terminatedearlier by the operation of an additional dial-pushbutton.

Accordingly, one feature of the invention involves the generation of aguard tone dial identifying signal following the generation of eachmultifrequency dial pulse by the dial of a coin operated telephone.

Another feature relates to the shorting out of the receiver during guardtone generation in order to prevent the customer from hearing the guardtone.

A further feature pertains to an arrangement for isolating the guardtone oscillator from variations in loop impedance.

The principles of the invention together with additional objects andfeatures thereof will be fully apprehended from the following detaileddescription of an illustrative embodiment and from the appended drawingin which:

FIG. 1 is a block diagram of the circuitry of a coin operated telephonein accordance with the principles of the invention;

FIG. 2 is a schematic circuit diagram of a portion of the circuitryshown in block form in FIG. 1;

FIG. 3 is a sketch of a multifrequency signal generating pushbuttonoperated dial of the type used in a coin operated telephone set inaccordance with the invention;

FIG. 4 is a schematic circuit diagram of the guard oscillator controlcircuit shown in block form in FIG. 1 and is a modified version of thatcircuit as shown in FIG. 2;

. FIG. 5A is a sketch of the type of signal train developed by a cointelephone in accordance wih the invention during a normal dialingsequence; and

FIG. 5B is a sketch of the type of signal train developed by a cointelephone in accordance with the invention during a fraudulent attemptto simulate a coin deposit signal representing a 10 cent deposit.

FIG. 1 indicates the general functional relationship between theelements required in a coin telephone set in accordance with theinvention and the elements of a conventional set. The conventionalsubscribers loop includes tip lead T and ring lead R. A guard signaloscillator circuit 102 is in series relation with tip lead T and may beshunted by a control path 103 under the supervision of a guardoscillator control circuit 101. Guard oscillator control circuit 101 isalso in series with tip lead T. The rernaining elements or circuitblocks shown in FIG. 1 are conventional and include a coin signaloscillator control circuit 104, a coin signal oscillator 105, logiccircuitry 106 for controlling the overall sequencing of the telephoneoperation and the speech network 107. Each of these last named circuitblocks may take any one of a number of prior art forms including, forexample, the circuitry shown in Patent 3,146,312 issued to E. R. Andreggand L. A. Strommen on Aug. 25, 1964 or in application Ser. No. 454,889filed by A. Busala, R. R. Stokes and L. A. Stromrnen on May 11, 1965.

Details of the guard oscillator control circuit 101 and its associatedcontrol path 103 are shown in FIG. 2. In FIG. 2 prior art elements suchas those shown in FIG. 1 are lumped together in a single block 201labeled telephone set circuitry, shown to the right of dotted line 202.In order further to clarify the relationship between the circuitry of acoin telephone set that embodies the features of the invention and priorart circuit arrangements, the conventional line current detecting relayA, its associated shunting varistors VR3 and VR4 and the shuntingcapacitor C2 are also shown in detail.

In accordance with the invention, the guard oscillator control circuit101 includes a relay coil B in the collector circuit of a controltransistor Q3. Varistors VR1 and VR2 provide an A-C shunting path aroundguard oscillator control circuit 101, serving to isolate this circuitfrom variations in loop impedance. The operation of transistor Q3 is inturn controlled by a timing circuit comprising resistor R1 and capacitorC1. Transfer contacts S1 are operated by a common switch S, shown inFIG. 3, which in turn is operatively responsive to the depression of anyone of the pushbutton DPB of the pushbutton dial assembly 301. Detailsof an illustrative dial assembly of the type shown in FIG. 3 aredisclosed, for example, in Patent 3,109,071 issued to C. E. Mitchell etal. on Oct. 29, 1963.

The detailed operation of the circuit shown in FIG. 2 is as follows.Depression of any one of the pushbuttons DPB of dial 301 operates switchS and the associated make and brake contacts S1 in the manner shown.Brake contact S1 prevents relay B from operating. As a result, noidentifying tone is produced during the time that conventionalmultifrequency dial signals are present. A charging path for capacitorC1 is maintained during this period 'by way of make contact S1 andcapacitor C1 is charged by the current on the line. The release of theoperated one of the pushbuttons DPB allows switch S1 to return to itsnormal or nonoperated position and the operating path for relay B isthereby completed over nonoperated break contact S1. Relay B operatesand is held operated by current in the emitter-collector path oftransistor Q3. The current in relay B is in effect controlled by thedecaying base current of transistor Q3. Stated otherwise, the timeduring which relay B remains operated is controlled by the time constantintroduced by resistor R1 and capacitor C1 in the base circuit oftransistor Q3.

The operation of relay B operates transfer contacts B thus opening thecontrol path 103 that otherwise shorts the guard oscillator circuit 102.The operation of make contact B places a short across telephone circuit201. Consequently, tone from the guard oscillator 102 is applied to theline but not to the speech network, and as a result, the customer is notaware that a guard tone is being generated. This feature of theinvention deters a customer from interfering with the generation ofguard tone by oscillator 102, inasmuch as the customer is unaware thatsuch tone is being generated, and further, it avoids the otherwiselikely etfect of reducing dialing speed as the customer, if the guardtone were audible, would be inclined to wait after the dialing of eachdigit until the termination of the guard tone before dialing the nextsucceeding digit.

The dialing of successive digits by dial 301 in a telephone set inaccordance with the invention results in a train of signals such as thatillustrated in FIG. 5A. An attempt to simulate a coin deposit signalrepresenting a 10 cent deposit by fraudulent manipulation of the dialresults in a train of signals such as that illustrated in FIG. 5B.

The guard oscillator 102 may take any one of a number of forms such asthat of a conventional transistor oscillator for example. The frequencyof oscillator 102 may be selected, advantageously in accordance with theinvention, to provide maximum separation from the frequency range of theconventionally generated multifrequency dial signals and, preferably,the guard signals should be in a frequency range outside the effectiverange of the associated central ofiice multifrequency receiver.

Under certain conditions the guard oscillator control circuit 101, shownin detail in FIG. 2, is somewhat sensitive to changes in temperature andvariations in transistor parameters. Some improvement in these areas isachieved by a modified guard oscillator control circuit of the typeshown in FIG. 4. Circuit elements in FIG. 4 corresponding to circuitelements in FIG. 2 are similarly identified.

As indicated above, in the circuit shown in FIG. 2 a single transistorQ3 is employed to control current in relay B and the operate time ofrelay B is dependent on the transistor current falling below the releasecurrent level of the relay. Consequently, a circuit such as that shownin FIG. 2 is dependent for its operation on the [3 or amplification ofthe transistor. The circuit shown in FIG. 4 employs a somewhat differentprinciple of operation, however, in that the operate time of relay B iscontrolled primarily by a transistor in a cut-off state. The time ofswitching is determined by the time of turn-off of the transistor whichis, in turn, the time that the base to emitter voltage increases to avalue great enough to forward bias the base to emitter junction. Anadvantage obviously stems from the fact that the percentage change involtage with variations in temperature and transistor parameters issmall when compared to the percentage change in transistor gain.

A further understanding of the principles of the circuit shown in FIG. 4may be gained from following a detailed description of overall circuitoperation. In the static state, with the coin telephone set in use butwith no dial button depressed, transistor Q2 is saturated withtransistor Q1 off. Voltage V is maintained relatively constant over theentire range of line currents owing to the characteristics of varistorsVR1 and VR2. Capacitor C1 is charged to a voltage level that issubstantially equal to the emitter to base voltage of the saturatedtransistor Q2. I

When switch S1 is transferred to the off-normal position, which occursas described above when one of the dial pushbuttons DPB (FIG. 3) isdepressed, capacitor C1 charges through the saturated base junction oftransistor Q2 and is placed in parallel with resistor R1 and capacitorC3. Capacitor C1 thus charges to a voltage level approximately equal toV Inasmuch as transistor Q2 was already in a saturated state, there isno change in the conducting state of transistors Q1 or Q2 and no outputfrom the circuit is produced. As soon as switch S1 returns to its normalposition upon the release of a dial pushbutton, the voltage acrosscapacitor C1 is a negative voltage and reverse biases the base junctionof transistor Q2, allowing the transistor Q2 collector to emittervoltage to increase to a value that saturates transistor Q1. The voltageacross capacitor C1 increases from a level approximately equal to -Vtoward a value of +V at an exponential rate controlled by the timeconstant of resistor R1 and capacitor C1. The charge current for thisportion of the operation may be on the order of 40 microamps and wouldbe affected by any leakage current, I of the transistor. It is desirabletherefore to employ a silicon transistor with a very low :I TransistorQ2 again turns on when the voltage across capacitor C1 becomes greatenough to forward bias transistor Q2. Capacitor C3 is used to ensurethat the circuit will not trigger when the line current is momentarilyinterrupted.

It is to be understood that the embodiments described herein are merelyillustrative of the principles of the invention. Various modificationsmay be made thereto by persons skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

1. In a coin operated telephone, in combination, a plurality of digitindicating means, means responsive to the operation of any one of saidindicating means for generating a dial signal indicative of acorresponding digit in terms of frequency composition, and meansresponsive to the release of any one of said indicating means from anoperated condition for generating a guard signal of a common frequencycomposition, said last named frequency composition being substantiallydifferent from and readily distinguishable from any one of saidfrequency compositions corresponding to said digits, thereby reducingthe possibility of confusion between dial generated signals and coinidentification signals.

2. Apparatus in accordance with claim 1 wherein said digid indicatingmeans comprises a pushbutton type telephone dial.

3. Apparatus in accordance with claim 1 wherein said guard signalgenerating means comprises a single oscillator.

4. In a coin operated telephone, in combination, an array of digitindicating dial pushbuttons, first means responsive to the depression ofeach of said pushbuttons for generating a multifrequency signalindicative of a corresponding digit in terms of a frequency code, andmeans responsive to the release of any one of said pushbuttons from adepressed condition for generating a common single frequency guardsignal immediately following the signal generated by said first means,thereby reducing the possibility of confusion between signals generatedby said first means and coin identification signals.

5. A coin operated telephone circuit comprising, in combination, anarray of digit indicating dial pushbuttons, means responsive to thedepression of each of said pushbuttons for generating a multifrequencysignal indicative of a corresponding digit interms of a frequency code,an oscillator, common switch means operatively responsive to the releaseof any one of said pushbuttons for energizing said oscillator thereby tocause the generation of a common single frequency guard signalimmediately following the generation of any one of said multifrequencysignals, whereby the possibility of confusion between signals generatedby said dial and signals generated by the deposit of coins issubstantially reduced.

6. Apparatus in accordance with claim 5, including a subscribers loop, arelay in series with one side of said loop, a timing circuit in serieswith said relay, an oscillator in series with said relay, said switchmeans including a first switch responsive to the depression of any oneof said pushbuttons for completing a first shunt path around said relay,said relay including contacts operatively responsive to said relay forcompleting a second shunt path around said oscillator, and said firstswitch opening said first shunt path upon the release of any one of saidpushbuttons from an operated condition, whereupon said relay operates,opening said second shunt path and applying said guard signal acrosssaid loop.

7. Apparatus in accordance with claim 6 wherein said timing circuitincludes a transistor switch having base, emitter and collectorelectrodes, the collector-emitter path of said transistor being inseries with said relay, a resistive element connected between the baseof said transistor and said first shunt path, and a capacitive elementin said first shunt path.

8. Apparatus in accordance with claim 7 including at least one varistorin a shunt path across both said relay and said transistor.

9. A coin operated telephone circuit comprising, in combination, apushbutton dial assembly including means responsive to the depression ofeach digit designating pushbutton for generating a multifrequency signalburst indicative of the corresponding digit in terms of a frequencycode, a subscribers loop, an oscillator in series relation with one sideof said loop, circuit means connected across said loop responsive to therelease of one of said pushbuttons from a depressed condition forenergizing said oscillator for a preselected period, whereupon a singlefrequency guardsignal of preselected duration is applied to said loopfollowing the generation of each of said multifrequency signals, saidcircuit means including means responsive to the depression of one ofsaid pushbuttons for terminating the generation of one of said guardsignals prior to the end of said duration, thereby preventing thesimultaneous generation of one of said multifrequency signals and one ofsaid guard signals, thereby reducing the possibility of confusionbetween signals generated by said dial assembly and coin depositidentification signals.

10. Apparatus in accordance with claim 9 wherein said circuit meansincludes a relay having a coil bridged across said loop, first andsecond capacitive elements in parallel with said coil, a resistiveelement shunting one of said capacitive elements, a first transistorswitch having base, emitter and collector electrodes with its collectorto emitter path in series relation with said coil, a conductive pathincluding a resistive element bridged across said loop in parallelrelation with said coil and in parallel relation with said capacitiveelements, a second transistor switch having base, emitter and collectorelectrodes with its collector to emitter path in series relation withsaid conductive path and means connecting said base electrode of saidfirst transistor to said collector electrode of said second transistor.

References Cited UNITED STATES PATENTS 3,069,502 12/1962 Edstrom et al.l79--6.3

WILLIAM C. COOPER, Primary Examiner.

J. S. BLACK, Assistant Examiner.

